1. Field of the Invention
The present invention relates to a thin-film transistor (“TFT”) array substrate for an X-ray detector and the X-ray detector having the TFT array substrate. More particularly, the present invention relates to the TFT array substrate for the X-ray detector capable of improving reliability and the X-ray detector having the TFT array substrate.
2. Description of the Related Art
A scintillator converts an X-ray which is emitted from an X-ray source and passes through an object according to the density of the object, into green light which is a visible ray. Diagnostic radiography is a conventional method through which an amount of electrical charge is detected when the light having passed through the scintillator, passes through a light-conversion sensor and the amount of the electrical charge is converted into a digital signal. Diagnostic radiography is classified into an image processing method and a storage method, or an analog-type and a digital-type diagnostic radiography.
Analog-type diagnostic radiography uses a photosensitive film as media on which an image is stored. For example, a contrast ratio is changed according to a difference of brightness which develops according to a level of exposure by using the image stored on the photosensitive film.
Digital-type diagnostic radiography is classified into computed radiography (“CR”) and digital radiography (“DR”).
CR directly stores an image emitted from a scintillator by using a magnetic material or a fluorescent material as an image storage sensor, and then displays the image on a display device by processing the image through a scanner. CR uses a digital image processing method, but does not substantially display a digital image because CR uses a scanner. For example, CR includes limitations in that digital image processing has limited editing functions, simultaneous transmitting functions, and the ability to achieve high contrast ratios.
On the other hand, DR is used for basic image processing in a medical diagnostic device. DR is classified into charge-coupled device (“CCD”) DR, complementary metal-oxide semiconductor (“CMOS”) DR and flat panel (“FP”) DR according to a type of sensor that converts the green light emitted from the scintillator.
The three types of DR use the same basic principles for digital image processing. For example, after a visible ray, emitted from an X-ray source and passing through an object and a scintillator is classified according to pixel, and is incident into a DR detecting sensor, a light-conversion apparatus stores amounts of electrical charge into each pixel. Then, an analog-to-digital converter (“ADC”) converts the amounts of electrical charge stored in each pixel into a digital signal, to process a digital image.
CCD and CMOS DR use relatively small sensors, so that an image has to be enlarged when performing digital image processing. Thus, projection-type DR has disadvantages with regard to resolution, brightness, contrast ratio and so on. CCD and CMOS DR correspond to methods used in large-size televisions, such as digital light processing (“DLP”)-type televisions and liquid crystal display (“LCD”)-type televisions.
A DR method using flat panel glass produces an X-ray image having the best quality, by allowing matching with a photoelectric sensor at a 1:1 ratio. Flat panel X-ray detecting (“FPXD”)-type DR is the most advanced type of DR. FPXD DR is classified into a direct-type and an indirect-type.
Direct-type DR directly biases a photoelectric charge which is converted by a scintillator to a thin-film transistor (“TFT”) circuit by using the TFT circuit, and induces the photoelectric charge in the TFT circuit, to induce a photoelectric current. Then, the photoelectric current is processed into a digital image through an ADC. For example, direct-type DR uses a simple structure in which a scintillator having amorphous selenium (“a-Se”)-based material directly attached to the TFT circuit and the TFT circuit are directly biased. However, direct-type DR has a disadvantage in that the scintillator having the amorphous selenium-based material which includes high photoelectric conversion efficiency has to be directly coated on the TFT circuit.
In addition, the scintillator having the amorphous selenium-based material that has the high photoelectric conversion efficiency is vulnerable to heat generated in operating the TFT circuit, so that the detection ability of the scintillator may be deteriorated.
Indirect-type DR includes the use of an X-ray detector. Each pixel of the X-ray detector includes a p-type photoconductive layer implanted with p-type impurities, a photoconductive layer having no impurities, an n-type photoconductive layer implanted with n-type impurities, and a positive-intrinsic-negative (“PIN”) photodiode having two electrodes to apply a voltage to both ends of the conductive layer. A scintillator is formed on the structure. The X-ray detector applies electrons that are generated from the PIN photodiode by an externally irradiated X-ray, and generates a bias voltage to an outside, to convert the light into an electrical signal.
Particularly, in indirect-type DR, the light emitted from an X-ray source passes through an object and is converted into a green light in a cesium iodide: thallium (“CsI:TI”) scintillator. Then, when the light is incident into an intrinsic silicon layer of a PIN junction diode on a flat panel, electrons and holes are generated in the silicon. An upper layer of a p-type silicon (“p-Si”) layer is biased by a negative voltage between approximately 5 V and approximately 7 V, the electrons of a negative charge move toward the direction of an n-type silicon (“n-Si”) layer different from that of the p-Si layer. The electrons moved to the n-Si layer pass through a source-drain layer on the TFT substrate and are integrated on a read-out integrated circuit (“ROIC”). A gate layer of the TFT circuit is driven to read the integrated electrons.
A signal read from each pixel as mentioned above, is an analog signal having a photocurrent unit. The analog signal is different according to a quantity of light incident into each pixel unit. For example, the intensity of an X-ray incident into the scintillator differs according to the density of the object. The analog signal that differs according to the quantity of light and the pixel unit, is digitized through the ADC, so that a digital image is displayed on a screen.
The photoelectric conversion efficiency of a PIN photodiode, and the efficiency of a scintillator converting an X-ray into a visible ray are limited. In addition, leakage current may occur in the X-ray detector.